1. Field of the Invention
The presently disclosed subject matter relates to a light guide unit, a light guide unit assembly, and a lighting device including the same.
2. Description of the Related Art
A lighting device that radiates light from an elongate rectangular light-emitting surface (radiating surface) is conventionally known. An example of this type of lighting device is described in Japanese Patent Laid-Open Publication No. 2003-317508 (hereinafter, this type of lighting device is referred to as a “conventional lighting device”).
More specifically, FIGS. 3 and 4 of the above-referenced Japanese reference publication show a conventional lighting device in which a first reflecting surface reflects light emitted from a light source toward a direction perpendicular to the optical axis of the light source. A second reflecting surface is provided to surround the light source in order to efficiently use the light emitted from the light source. The second reflecting surface reflects the light emitted from the light source toward a direction along the optical axis of the light source.
That is, in the conventional lighting device described above, the light emitted from the light source is directed along the optical axis of the light source directly or by the second reflecting surface. This light is then reflected by the first reflecting surface toward the direction perpendicular to the optical axis of the light source. Consequently, the light is radiated from an entire surface of the elongate rectangular light-emitting surface (radiating surface) of the conventional lighting device. The radiating surface is longer in the optical-axis direction and is shorter in the direction perpendicular to the optical axis (i.e., in a diameter direction of the second reflecting surface).
In this conventional lighting device, a length of the shorter side of the rectangular light-emitting surface (radiating surface) is set to be relatively larger, i.e., approximately as large, as a diameter of the second reflecting surface. Thus, the conventional lighting device can use the light emitted from the light source relatively efficiently. However, in this structure, if the length of the shorter side of the light-emitting surface (radiating surface) is set to be smaller than the diameter of the second reflecting surface in order to make the light-emitting surface (radiating surface) more elongate, it may not be possible to use a part of the light emitted from the light source (which part is emitted from right and left sides of the light source when the light source is seen from a radiation direction). In other words, only (or substantially only) a central part of the light that corresponds to a width of the shorter side of the light-emitting surface can be used. Therefore, light-use efficiency may be significantly reduced.